CD45 or Leukocyte Common Antigen (LCA/T200) belongs to a family of membrane sialoglycoproteins with MW of 180kDa-220kDa. Human CD45 contains three exons which encode peptide segments designated A, B and C, respectively. The differential splicing of the exons generates various isoforms. CD45RA. is a single chain glycoprotein. It is expressed on approximately 40-50% of peripheral CD4+ T- cells, 50% of peripheral CD8+ T-cells, B-cells, and leukemic B-cell lines. T-cells expressing CD45RA and CD45RO define complementary, predominantly non- overlapping populations of resting peripheral T-cells. CD45RA are naive or virgin T-cells while, T-cells expressing CD45RO are memory T-cells.